1. Field of the Invention
The present invention relates to an autobalance hybrid circuit useful in a telephone system to interconnect a 4-wire telephone line having separate receive and transmit pairs to a 2-wire duplex telephone line, and automatically to compensate for changes in impedance of the 2-wire line.
2. Description of the Prior Art
In telephone systems, the interconnection between a central office and a local switchboard typically employs a 4-wire telephone line in which separate pairs of wires are used for the transmit and receive signals. However, the local switchboard and telephone instruments typically use 2-wire lines in which the same pair of wires is used to carry both the incoming and outgoing audio signals. Interconnection between 4-wire and 2-wire lines is accomplished by using a hybrid coil circuit.
The basic arrangement of a hybrid-coil 2-wire to 4-wire terminating set is well known, and is described e.g., in the Electronics Engineers' Handbook, Donald G. Fink, Editor-in-Chief, McGraw-Hill, 1975, in section 22-32. The hybrid is essentially a bridge circuit employing two transformers. One transformer is connected to the 2-wire telephone line, and the other is connected to a balancing network having an impedance which matches that of the 2-wire line as closely as possible over the frequency range of the transmitted audio. When a signal enters the hybrid from the 4-wire receive line, it divides equally between the 2-wire circuit and the network; if these are in balance, no audio signal is sent back to the 4-wire line transmit pair.
If the balance is disturbed, some of the received signal is reflected back to the 4-wire transmit line, and is perceived as an echo. The ratio in decibels between received and reflected power is known as the return loss of the hybrid circuit. If the return loss is low, that is, if a substantial part of the signal is reflected back to the transmit line, objectionable feedback, "singing" or "howling" effects can occur.
In prior art hybrid balancing networks, fixed circuits have been employed using component values selected to match ideal 2-wire line impedances. Such an arrangement represents a compromise that may be acceptable where the 2-wire line impedance is known, and remains constant. Alternatively, manually adjustable balancing networks have been employed, which are optimized for a particular 2-wire line when the hybrid coil is installed. These arrangements are not satisfactory, however, in a situation where the 2-wire line impedance may change substantially from time to time. A principal object of the present invention is to provide a hybrid circuit which dynamically compensates for changes in the 2-wire line impedance.
One example of an application of such an "autobalance" hybrid transformer circuit is in connection with a private telephone exchange of the type where numerous telephone instruments and/or other telephone equipment may be connected at different times to the same 2-wire line. In such a system, at certain times only two persons may be speaking to each other. At other times, three, four or more telephone instruments may be connected to the same 2-wire line. Public address apparatus also occasionally may be connected to such line. Under various of these conditions, the effective impedance of the 2-wire line, as seen by the hybrid, may differ substantially. Howling, singing or other objectionable feedback effects may occur at unpredictable times, when various combinations of individual phone instruments are connected to the same line. The inventive autobalance hybrid transformer circuit is intended to compensate automatically for these changes in impedance, and thereby totally eliminate the problem of howling or singing.
To this end, another objective of the present invention is to provide a hybrid circuit wherein the balancing network is dynamically adjusted to match the impedance of the 2-wire line, thereby maximizing the return loss of the circuit regardless of changes in the 2-wire line impedance. A further object is to achieve such operation by comparing the envelope voltages separately developed across a 2-wire line coupling transformer and a balancing network coupling transformer, and using this difference voltage to adjust the balancing network until a null is achieved.
A further object of the present invention is to provide a unique rectifier envelope detector having a low threshold, and particularly useful in an autobalance hybrid circuit. Yet another objective is to provide a unique combination squelch and automatic gain control (AGC) circuit for use with such a hybrid transformer.